Mindanao State University – Iligan institute of Technology College of Engineering
Department of Chemical Engineering and Technology
Ruel B. Cedeño
BSChE - IV
Engr. Rodel D.Guerrero
The word electrolysis comes from the Greek [ɛ̌ːlektron] meaning "amber" and [lýsis] "dissolution". The discovery of electrolysis began in 1785 when Martinus van Marum's electrostatic generator was used to reduce tin, zinc, and antimony from their salts using direct current.
In 1800, William Nicolson and Johan ritter was able to decompose water into hydrogen and oxygen in such process.
In 1807, potassium, sodium, barium, calcium, and magnesium were discovered by Sir Humphry Davy using electrolysis. Decades later, Paul Emile Locoq de Boisbaudran discovered gallium in 1875 using similar procedure. In 1886, Fluorine was discovered by Henri Moissan by applying the same principle. In 1886, the Hall-Heroult process was developed for making aluminum which is still widely used today. Eventually, Castner-Kellner process was developed for making sodium hydroxide in 1890.
I. ALUMINUM INDUSTRY A. USES
B. RAW MATERIALS
The raw materials needed to produce 1 kg of aluminum, the following relative amounts of raw materials are necessary: 4 kg bauxite, 0.415 kg carbon, 20 g aluminum fluoride, 2 g cryolite, and a supply of 13460 kWh of electrical energy.
Bauxite contains 40 to 60 mass% alumina combined with smaller amounts of silica, titania and iron oxide. Bayer process dissolves the aluminum component of bauxite ore in sodium hydroxide (caustic soda); removes impurities from the solution according to the reaction which occurs in the digester
2NaOH + Al2O3.xH2O 2NaAlO2 + (x+1)H2O
The resulting sodium aluminate is then acidified to precipitate aluminum hydroxide in the precipitator with this reaction
NaAlO2(aq)+ HCl(aq) Al(OH)3 (s) + NaCl (aq)
Aluminum hydroxide that precipitate out is heated in the rotary kiln to produce alumina according to this reaction
Al(OH)3 1/2 Al2O3 + 3/2 H2O
Step 2: Hall-Heroult process. This is the process of producing aluminum from electrolysis of Alumina. Aluminum cannot be produced by the electrolysis of an aqueous aluminum salt because hydronium ions readily oxidize elemental aluminum. Although a molten aluminum salt could be used instead, aluminum oxide has a melting point of over 2,000 °C (3,630 °F) so electrolyzing it is impractical. In the Hall–Héroult process alumina, Al2O3, is dissolved in molten cryolite, Na3AlF6 and electrolyzed. Thus, liquid aluminum is produced by the electrolytic reduction of alumina (Al2 O3) dissolved in an electrolyte (bath) mainly containing Cryolite (Na3AlF6). The overall chemical reaction can be written as:
2 Al2O3 (dissolved) +3C (s) 4 Al (l) +3 CO2 (g)
Pure cryolite has a melting point of 1,012 °C (1,854 °F). With a small percentage of alumina dissolved in it, its melting point drops to about 1,000 °C (1,830 °F). Aluminum fluoride, and calcium fluoride are added to the mixture to further reduce the melting point.
Anode Reactions: During the electrolysis reaction gaseous CO2 evolved. The carbon is provided by the anode material, the oxygen is transported to the anode in the form of AL-O-F complex anions. At high alumina concentrations the species Al2O2F42-and Al2O2F64- may be discharged as suggested by the reactions:
Al2O2F42- + 4 F- + C CO2 + 4e- + 2 AlF4- Al2O2F64- + 2F- + C CO2+ 4e- + 2AlF4-
Cathode Reactions - The only cation present in cryolite-alumina melts is Na+. Despite Na+ being the
main current carrier, it has been showed that formation of aluminum is favored over sodium in the electrolyte compositions used industrially, since the reversible EMF is favorable. That is aluminum is the thermodynamically preferred product. As there is no evidence that Al3+ ions are present, all of the aluminum in the melt is bound in different anionic complexes. Al-O-F takes part in the anode reactions so the most probable cathode reactions involve the remaining aluminum – containing ions AlF63- and AlF4-. The overall reaction can be written as
AlF63- + 3e- Al + 6F -AlF4- + 3e- Al + 4F- OVERALL 2 Al2O3 + 3C 4 Al + 3 CO2 Anode: 6 O2- + 3C 3CO 2 + 12 e -Cathode: 4 Al3+ + 12 e- 4 Al
The industrial production of aluminum uses a current of 25 000 amps. Calculate the number of hours required to produce 10 kg of aluminum from the electrolysis of molten aluminum oxide. ( 1 mole e- = 96500 A ∙ s )
Al3+ + 3e- Al
10 kg Al ( 1 kmol Al / 27kg )(3 kmol e-/1 kmol Al)(96500 A ∙ s/1 mol e-)(1/25000 A)(10000)(1 hr/3600s) = 1.19 hr
II. SODIUM INDUSTRY
Sodium is a very reactive metal first discovered by Sir Humphry Davy in 1807. This element is extracted by electrolysing molten sodium chloride in the Down's cell.
Sodium is used for the production of sodium borohydride, sodium azide, indigo, and triphenylphosphine. Furthermore, it can be utilized as alloying metal, an anti-scaling agent, and as a reducing agent for metals when other materials are ineffective. Sodium vapor lamps are often used for street lighting in cities and give colours ranging from yellow-orange to peach as the pressure increases. It is also used as a desiccant; it gives an intense blue colouration with benzophenone when the desiccate is dry. It also play an important role in organic synthesis since it is widely used in various organic reactions such as the Birch reduction, and the sodium fusion test which is conducted to qualitatively analyze compounds. In a more advanced technology, it is used to create artificial laser guide stars that assist in the adaptive optics for land-based visible light telescopes.
The production of metallic sodium begins when the molten sodium chloride is sent to the down cell. The chloride ions are attracted to the anode, where they lose electrons and form chlorine gas according to the following reaction
2Cl– (l) Cl
2 (g) + 2e–
The positive sodium ions are attracted to the cathode. They gain electrons to form molten sodium metal. Na+ (l) + e– Na (l)
The overall reaction which takes place in the cell is:
2 NaCl (l) 2 Na (l) + Cl2 (g)
The cathode is a circle of steel around the graphite anode. At 600°C sodium and chlorine would react violently together to reform sodium chloride. To pre-vent this from happening, the Down's cell contains a steel gauze around the graphite anode to keep it and the cathode apart. The molten sodium floats on the electrolyte and is run off for storage.
A problem arises, however, in that calcium ions are also attracted to the cathode, where they form calcium metal. Therefore, the sodium which is run off contains a significant proportion of calcium. Fortunately, the calcium crystallises out when the mixture cools and relatively pure sodium metal remains.
III. MAGNESIUM INDUSTRY
Magnesium has a variety of usage which includes flash photography, flares, pyrotechnics, fireworks and sparklers. It is also used in organic reactions as reducing agents as well as an additive agent in conventional propellants.
B. RAW MATERIALS
The main sources of magnesium compounds are: seawater (magnesium chloride, MgCl2) and minerals such as dolomite (CaCO3·MgCO3), magnesite (MgCO3), carnallite (KCl·MgCl2·6H2O).
Dolomite rock is crushed and heated in the kiln which will then undergo precipitation upon addition of NaOH according to the reaction
MgCO3 + 2 NaOH Mg(OH)2 + Na2CO3 and MgCl2 + NaOH Mg(OH)2 + NaCl
The precipitate passes through filters where impurities are removed and it is then reacted with HCl in the neutralizer according to the equation
Mg(OH)2 + 2 HCl MgCl2 + 2 H2O
The resulting magnesium chloride is dried and dehydrated from 35% to 73% MgCl2. It will then proceed to the electrolytic cells which will produce the net following reaction
The major chemical reactions in the process are the following MgCl2 Mg + Cl2
IV. CHLORATES AND PERCHLORATES INDUSTRY A. USES
Chlorates and Perchlorates are used to treat thyroid disorders, additive in pyrotechnics industry, and also as a component of solid rocket fuel.
B. RAW MATERIALS Sodium chloride C. PRODUCTION
Aqueous solution of sodium chloride (brine) or seawater is sent to the settling tank where the suspended impurities are allowed to settle at the bottom which will be filtered later on. The purified NaCl is then sent to the electrolytic cells where it undergoes the following reaction
NaCl + 3H2O NaClO3 + 3H2 ClO3−(aq) + H2O(l) → ClO4−(aq) + H2(g)
The by-product hydrogen gas is processed and stored to a secure container. The liquid chlorate and perchlorate can then be directly supplied to the clients. Alternatively, if solid form is desired, the aqueous sodium chlorate is sent to a chiller where it is cooled down and then to the settling tank where it undergoes filtration or any method of separation. The product is dried which can then be supplied to the market.
V. ELECTROCHEMICAL CELLS
Primary Cells are not rechargeable and are discarded after they run down when all the chemicals are used up ie no more chemical potential energy available. Secondary Cells can be recharged after they have run down ie the discharge reactions producing the electricity are reversed to built up the store of chemical potential energy
A. Primary Cells
Galvanic cells in which the reactants are sealed in when manufactured and ready for immediate use i.e. the chemicals are capable of spontaneously reacting and the redox changes released energy as an electron flow (rather than heat energy). They cannot be recharged, and when they run down, that is the chemical reactants are completely depleted, they stop working and are discarded. The common ones such as the zinc– carbon batteries are used in torches, radios, cameras, flashlights, cameras etc.
In this cell, the anode discharging reaction is
Zn(s) + 4NH3(aq) ==> [Zn(NH3)4]2+(aq) + 2e– While the cathode discharging reaction is as follows
MnO2(s) + NH4+(aq) + e– ==> MnO(OH)(s) + NH3(aq) Hence, the overall working cell reaction is given by
Zn(s) + 4NH3(aq) + 2MnO2(s) + 2NH4+(aq)
B. Secondary cells are galvanic cells that must be charged before they can be used and rechargeable many times. In the charging process, the spontaneous–feasible cell reaction that produces electrical energy is reversed, so building up the chemical potential of the cell system. Example of this is the Car battery
In the anode, the discharging reaction is
Pb(s) + HSO4–(aq) ==> PbSO4(s) + H+(aq) + 2e– While in cathode, the reaction is given by
PbO2(s) + 3H+(aq) + HSO4–(aq) + 2e– ==> PbSO4(s) + 2H2O(l) Thus, the working cell reaction can be written as
PbO2(s) + 2H+(aq) + 2HSO4–(aq) + Pb(s) ==> 2PbSO4(s) + 2H2O(l)
Its advantages are as follows: Inexpensive, high power density (can car starter motor as well as lights), long shelf life, readily recharges, so has a long working life of many years. However, Lead needs to be recycled to avoid environmental contamination, sometimes generates hydrogen gas at the cathode when charging (explosive in air + spark) – though batteries seem to be made of a high standard these days in completely sealed units that last many years.
Another example is the Lithium-ion Battery which is a member of a family of rechargeable battery types in which lithium ions move from the anode to the cathode during discharge and back when charging. Li-ion batteries use an intercalated lithium compound as the electrode material,
The positive electrode half-reaction is:
LiCoO2 Li1-nCoO2 + nLi+ + ne- The negative electrode half-reaction is:
nLi+ + ne- + C Li nC
The overall reaction has its limits. Overdischarge supersaturates lithium cobalt oxide, leading to the production of lithium oxide, possibly by the following irreversible reaction:
Li+ + e- + LiCoO
2 Li2O + CoO
Part III - LATEST ADVANCEMENTS SPE and HTE Water Electrolysis
(Source: ENERGY CARRIERS AND CONVERSION SYSTEMS – Vol. I - Isao Abe, March 3, 2013)
New technologies of water electrolysis which are under development have more possibilities of improvement. These technologies are PEM ( polymer electrolyte membrane) water electrolysis and HTE (high temperature steam electrolysis).
A. High Temperature Steam Electrolysis (HTE)
High temperature electrolysis is more efficient economically than traditional room temperature electrolysis because some of the energy is supplied as heat is cheaper than electricity, and because the electrolysis is more efficient at higher temperatures. In fact, at 2500°C, electrical input is unnecessary because water breaks down to hydrogen and oxygen through thermolysis. Such temperatures are impractical; proposed HTE systems operate between 100°C and 850°C.
The efficiency improvement of high-temperature electrolysis is best appreciated by assuming the electricity used comes from a heat engine, and then considering the amount of heat energy necessary to produce one kg hydrogen (141.86 megajoules), both in the HTE process itself and also in producing the electricity used. At 100°C, 350 megajoules of thermal energy are required (41% efficient). At 850°C, 225 megajoules are required (64% efficient).
B. Solid Polymer Electrolyte Membrane Electrolysis
One of the largest advantages to PEM electrolysis is its ability to operate at high current densities. This can in result in reduced operational costs, especially for systems coupled with very dynamic energy sources such as wind and solar, where sudden spikes in energy input would otherwise result in uncaptured energy. The polymer electrolyte allows the PEM electrolyzer to operate with a very thin membrane (~100-200μm) while still allowing high pressures, resulting in low ohmic losses, primarily caused by the conduction of protons across the membrane (0.1 S/cm) and a compressed hydrogen output.
The polymer electrolyte membrane, due to its solid structure, exhibits a low gas crossover rate resulting in very high product gas purity. Maintaining a high gas purity is important for storage safety and for the direct usage in a fuel cell. The safety limits for H2 in O2 are at standard conditions 4 mol-% H2 in O.
Austin G. and Shreve R. (1984) Shreve’s Chemical Process Industries. McGraw Hill Publishing Co. Brooks G, Trang S, Witt P, Khan, MNH, Nagle M (2006) The Carbothermic Route to Magnesium. The
Journal of Minerals, Metals & Materials Society (JOM): 51-55.
Bushnell S. and Purkis P. (1984). Solid polymer electrolyte systems for electrolytic hydrogen production.
Chemistry and Industry 16 January, pp. 61–68. [This is the explanation of SPE water electrolysis system
developed by CJB, an engineering company in the UK, and also a good paper to get the general idea of SPE water electrolysis.]
Donitz W. and Erdle E. (1984). High temperature electrolysis of water vapor—status of development and perspective for application. Hydrogen Energy Progress V (Proceedings of the WHEC 5), 767–775. [This describes the electrolytic cell of HTE.]
Masakalick N. High temperature electrolysis cell performance characterization. Hydrogen Energy
Progress V (Proceedings of the WHEC 5), 801–811. [This describes the result of cell module testing of
HTE in detail.]
CSIRO (2006a) http://www.csiro.au/csiro/conte nt/standard/ps18r.html http://en.wikipedia.com/Aluminum